1. Field of the Invention
The instant invention relates to a highly-active silver catalyst useful in the epoxidation of ethylene to ethylene oxide and more particularly to a highly-active silver catalyst prepared by abrading the surface of a previously-prepared, promoted silver catalyst whereby about 1 to about 10 percent by weight of the catalyst is removed.
2. Description of the Prior Art
Supported silver catalysts have long been used for the air oxidation of ethylene and more recently in a so-called "oxygen process". Although the first reference to the use of silver as such a catalyst was made by Walter in British Pat. No. 21,941 (1905), it was not until some thirty years later that the first disclosures were made of the use of silver as a catalyst in the vapor phase oxidation of ethylene to ethylene oxide. See Societe Francaise De Lefort, U.S. Pat. No. 1,998,878 (1935).
A variety of techniques have been developed for the depositing of relatively small, but highly active amounts of silver on surfaces of non-silver support such as alumina. For example, McKim and Cambron in Canadian Journal of Research, Volume 27, Section B (1949) at 813-827, describe a method for depositing particulate silver on a support by decomposing silver oxalate in aqueous ethanolamine at 60.degree. C. and forming a paste which is applied to the surface of the support. In U.S. Pat. No. 3,043,854 issued July 10, 1962, to Endler, a silver coating formed by decomposition of a silver carbonate slurry is applied to a catalyst support surface.
It has been disclosed that supported silver catalysts can be prepared by impregnating a porous substrate with certain silver containing solutions and evaporating or decomposing the solutions to deposit silver on the substrate. U.S. Pat. No. 3,702,259 to Nielsen describes the use of an aqueous silver salt impregnating solution consisting essentially of a silver salt of carboxylic acid, an organic amine solubilizing/reducing agent such as ethylenediamine, a mixture of ethylenediamine or ethanolamine and ammonia or a mixture of ethylenediamine and ethanolamine. Van Bylandtlaan, in Belgium Pat. No. 808,278 (1974) employs an aqueous solution of hexamethylenetetramine with an ethylendiamine silver complex to deposit silver on an alumina support by decomposition. Additionally, it has been disclosed in Japanese Pat. No. 71/19,606 to Fujii et al that impregnation of inorganic supports with aqueous silver nitrate/alkanolamine complexes with subsequent thermal decompsition gives supported silver catalysts for ethylene epoxidation.
Recently it has been disclosed in British Pat. No. 1,413,251 to Nielsen and La Rochelle that certain alkali metals such as cesium and rubidium can be deposited on a refractory support coincidentally with the silver metal (U.S. Pat. No. 4,012,425) to form highly efficient oxide catalysts. Cesium-modified catalysts are also disclosed in U.S. Pat. No. 4,097,414 to Cavitt.
Despite the advances made in the art with regard to silver catalysts for the production of ethylene oxide there is a definite need for catalysts which exhibit an even higher oxidation selectivity to ethylene oxide.